Hypertrophic cardiomyopathy (HCM), characterized by hypertrophy and fibrosis of the left ventricle (LV) and septum, is the most common inherited heart disease. The gene MYBPC3 encoding cardiac myosin binding protein C (cMyBPC) accounts for approximately 40-50% of cases in which causative mutations are identified. Of special note is the high frequency of nonsense mutations resulting in truncated proteins in MYBPC3 relative to other HCM-linked genes. The most prevalent hypothesis for the primary disease mechanism in these cases is inadequate sarcomeric wild-type cMyBPC expression. However, such haploinsufficiency of cMyBPC has yet to be satisfactorily demonstrated, and the Day lab has published data from a large sampling of human HCM myocardial tissue suggesting this is not the case. Growing evidence, supported by findings of ubiquitin proteasome system (UPS) dysfunction in cell and animal models expressing truncated cMyBPC as well as in human HCM, indicates protein quality control (PQC) mechanisms play an important role. Cardiac PQC is an area of undervalued potential in which further elucidation of the complex pathogenesis of HCM may be made. Chaperone proteins are crucial to proper PQC, but little is known about their effects on myocardial physiology. Experimentally, knockout of inducible Hsp70 and its cochaperone Hspa4 both caused LV hypertrophy in mice. This research training plan will investigate a potential link between UPS dysfunction and interactions of truncated cMyBPC with the Hsp70 family of chaperones. The main hypothesis is that UPS dysfunction and proteotoxicity in cardiomyocytes expressing truncated cMyBPC can be alleviated by clearance of the mutant proteins via the protein triaging activity of Hsp70 chaperones. The specific aims of this proposal are to define physical and functional interactions between mutant cMyBPC and the Hsp70 family of chaperones and to determine the effects of promoting cMyBPC degradation by modulating Hsp70 activity or expression on UPS dysfunction and proteotoxicity in cardiomyocytes. These aims will be accomplished using a complementary battery of biochemical, cellular, and whole organism assays performed in both established and innovative new cell culture and animal models of HCM.